The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. It is therefore desirable to operate the turbines at the highest possible temperatures. For any engine cycle compression ratio or bypass ratio, increasing the turbine entry gas temperature produces more specific thrust (e.g. engine thrust per unit of air mass flow). However, as turbine entry temperatures increase, it is necessary to develop components and materials better able to withstand the increased temperatures.
This has led to the replacement of metallic components, such as shroud segments, with CMC components having higher temperature capabilities. To accommodate the change in material, however, adaptations to the components have been proposed. For example, EP 0751104 discloses a ceramic segment having an abradable seal which is suitable for use with nickel base turbine blades, and EP 1965030 discloses a hollow section ceramic seal segment. For improved strength and toughness, the CMC can be continuous fibre reinforced.
Gas turbine engine components often require sealing, e.g. to retain a back face air pressure, maintain cooling flows and protect specific fuel consumption (SFC). For example, FIG. 1 shows schematically a longitudinal cross-section though a seal segment 1 which, in use, is positioned radially adjacent shroudless aerofoil blades 2 of the rotor of the engine. A circumferential row of such seal segments forms a shroud ring for the rotor. Neighbouring segments can be sealed to each other by strip seals, which are metal strips located in slots formed in side faces of neighbouring segments. Dash-dotted line 3 in FIG. 1 indicates the line taken by such a strip seal from the front to the rear of the segment. The segments can be sealed to the outer casing 4 of the rotor via bird mouth seals 5 at front and rear races of the segments. Cooling air for the ring enters a space 6 formed between the segments and the casing.
The strip and bird mouth seals are suitable for use with metallic seal segments. In particular, such seals require the segments to have high tolerance surface finishes of the type that can be achieved with metallic components. However, a problem associated with continuous fibre reinforced CMCs is that they generally have a surface texture similar to a woven fabric, which is not a suitable sealing face. The CMC surface can be ground to a high tolerance, but porosity in the CMC can then reduce sealing efficiency.